I’m not sure the evidence of the proposed bias supports the type of ev-psych responses being offered.
The only cases I’m aware of underdog bias actually mattering are of the Israel-Palestine type, not the Zug-Urk type. I-P poses no significant costs or benefit to the individual. Z-U poses tremendous costs or benefits to the individual. I don’t imagine I-P type support decisions meaningfully affect reproductive success. Unless there’s evidence that people still side with the underdog when it really costs them something, these ev-psych explanations seem to be explaining something that doesn’t happen.
I would posit that it’s cultural, and it’s fictional availability bias. In all of our stories, the underdog is invariably the good guy. It seems very difficult to tell a story about good giant multinational corporation beats evil little old lady. The reverse has been quite successful. Consequently, we tend to side with the underdog because we generalize from a great deal of fictional evidence that “proves” that the underdog is the good guy. This also explains why we stick with an underdog even when he ceases to be an underdog, as this is a typical pivot point in a story.
This raises the question of why this kind of story is so successful, which I admit I don’t have a great answer to.
He didn’t give you his word that you are wrong. He stated that your claim was not rigorous and that crucial parts of it required supporting evidence that you failed to provide.
He also claimed that you do not understand evolutionary psychology. (Edit) You have provided no evidence to dispute this claim. Of course you are probably not making an evpsych argument, so this comment is probably not necessary, but if it’s wrong and you are, you might consider rebutting it.
Rational responses to this would include providing evidence in support of your claim or explaining how a this predisposition might form. “Maybe we just don’t like overdogs” explains exactly nothing, except that we don’t like overdogs, which has already been stated.
Conversations are important, but making statements with no evidenciary claim and, well, no claim that adds meaning, are not.